The present invention relates to a frame step-out detecting system in data transmission and, more particularly, to a frame step-out detecting system in a transmission system using a convolutional code.
In a digital communication system, the transmission speeds (bit rates) of N parallel channels connected to both ends of a transmission path are an integer multiple of 2,400 Hz, respectively. The frame clock of a multiplexer and a demultiplexer for connecting these low-rate channels to a modulator and demodulator have a frequency 2,400 Hz. When the transmission path of the communication system has a transmission speed of 14.4 kbps (kilo bit per second) or less, the modulation and demodulation speeds of the MODEM are 2,400 Hz, and the modulator and the demodulator use clock signals commonly with the corresponding multiplexer and the demultiplexer, respectively. In this case, the modulation clock of the transmission side and the demodulation clock of the reception side are synchronized when the demodulator receives data correctly. At the same time, the frame clocks of the transmission side multiplexer and the reception side demultiplexer are also automatically synchronized. Therefore, no particular synchronizing means is needed.
When the transmission speed is larger than the above value, however, e.g., when the system has a transmission speed of 19.2 kbps, the modulation and demodulation speeds are 19,200.div.7=2,742.85 Hz since transmission is performed in units of 7 bits, and the modulator and the demodulator cannot use clock signals commonly with the corresponding multiplexer and the demultiplexer, respectively. In this case, a synchronizing means is needed since the multiplexer and the demultiplexer are not automatically synchronized with each other even when the modulator and the demodulator are synchronized with each other.
Conventionally, a frame step-out detecting means in data transmission detects frame step-out by inserting a frame sync signal having a specific pattern into a data string to be transmitted. Since such conventional frame step-out detecting system inserts the frame sync signal which does not have data to be transmitted, the data transmission speed is always lower than the transmission speed in a transmission path. This poses a critical problem in a system wherein data transmission is performed with a maximum communication capacity of a transmission path, as in data transmission using a telephone network.